Field of the Invention
The present invention relates to an electrographic image forming apparatus, an image forming system, and a concentration unevenness detecting method.
Description of the Related Art
In general, an image forming apparatus (such as a printer, a copying machine, and a facsimile) for using electrophotographic processing technology forms an electrostatic latent image by irradiating (expose) a charged photoreceptor with a laser beam based on image data. The electrostatic latent image is visualized and a toner image is formed by supplying a toner to a photoreceptor drum, on which the electrostatic latent image is formed, from a developing device. In addition, after the toner image has been directly or indirectly transferred to a paper sheet, the toner image is fixed by heating or pressurizing with a fixing nip, and accordingly, the image is formed on the paper sheet.
Further, an image forming system has put into practical use in which the former stage of the image forming apparatus is connected to a paper feeder and the latter stage is connected to a paper ejector. The paper feeder feeds a continuous paper sheet (referred to as “long paper sheet” below) such as continuous rolled paper and folded paper, and the paper ejector stores the long paper sheet on which the image has been formed by the image forming apparatus.
There is a problem in that, in this image forming apparatus, the image quality of the output image (image formed on the paper sheet) is deteriorated by deterioration of the photoreceptor drum, developer, and the like with time, the ambient environment of the device (change of temperature and humidity), and the like. Specifically, a phenomenon occurs in which colors of the input image are not faithfully reproduced in the output image and a color tone of the image is different from that of the other image. The image forming apparatus in the related art performs image stabilization control so that color reproducibility and color stability are secured.
In the image stabilization control, for example, an optical sensor detects the concentration of a patch image (toner pattern) formed on the photoreceptor drum, and the feedback of the detection result is reflected to an image forming condition such as a charged potential, a developing potential, and an exposure amount. Accordingly, the concentration of the image is corrected. Generally, the image stabilization control is regularly performed by using a non-image forming region when the image is continuously formed on the plurality of paper sheets.
Further, in the image forming apparatus, there is a case where concentration unevenness in the circumferential direction (vertical scanning direction) is generated in the toner image formed on the photoreceptor drum. The concentration unevenness is caused by change of the distance between the photoreceptor drum and a developing roller due to rotational deflection of the developing roller and humidity unevenness in the rotary axis direction of the photoreceptor drum. In this case, in the image formed on the paper sheet, the concentration unevenness is generated by synchronizing with the rotation cycles of the developing roller and the photoreceptor drum. The humidity unevenness in the axis direction of the photoreceptor drum is generated as follows. When the image forming apparatus is left stopping for a long time under high temperature and high humidity environment, air circulation in the image forming apparatus is lowered, and the humidity in a certain part on the photoreceptor drum becomes higher. Accordingly, the charge cannot be normally performed, and the humidity unevenness is generated. JP 2014-116711 A and JP 2013-195586 A disclose an image forming apparatus which can form an image with high quality by preventing this periodic concentration unevenness.
In the technique disclosed in JP 2014-116711 A, a predetermined print device, of which the concentration unevenness in the vertical scanning direction is a target to be corrected, continuously prints a plurality of test charts to measure the concentration unevenness in the vertical scanning direction. Profiles of the measured data obtained by optically reading them are connected in a print order in consideration of a paper gap. The connected data is divided into a plurality of pieces of data for each specific cycle, and effective data at all positions on the specific cycle in the plurality of pieces of data is averaged, and the averaged data is analyzed. According to the analysis, the concentration unevenness in the vertical scanning direction is detected, and correction data to remove the concentration unevenness in the vertical scanning direction generated in each specific cycle is created.
In the technique disclosed in JP 2013-195586 A, an image forming apparatus has a pattern forming unit which forms concentration change detecting patterns having a plurality of generation cycles on an endless belt along the conveying direction of the endless belt, a concentration sensor which detects the concentration change detecting pattern and outputs a concentration signal including information on a change of the concentration in the conveying direction of the endless belt, and a cycle detecting sensor which detects a plurality of cycles included in the change of the concentration.
However, regarding the technique disclosed in JP 2014-116711 A, it is necessary to separately print a test chart during normal image forming processing, and accordingly, the decrease of productivity has been a problem. On the other hand, the technique disclosed in JP 2013-195586 A is different from that in JP 2014-116711 A. It is not necessary to print the test chart during the normal image forming processing. However, since the concentration unevenness is generated at various cycles such as a cycle of the developing roller and a cycle of the photoreceptor drum, it is necessary to use a paper gap having a certain size (a region between an image forming region where the toner image to be transferred to a single paper sheet is formed and another image forming region, and a region where the concentration change detecting pattern is formed) to detect the concentration unevenness. The concentration unevenness cannot be sufficiently detected according to the size of the paper gap. Further, when the concentration unevenness is detected in the image forming system for forming the label image on the long paper sheet (label roll), it can be considered to form the concentration change detecting pattern in a margin instead of the paper gap. However, since the label images are formed on the long paper sheet at regular intervals, it is necessary to form the concentration change detecting patterns at regular intervals. There are many cases where the size of the concentration change detecting pattern which can be formed in the margin is not large enough to detect the concentration unevenness at one time.